Alloy



ALLOY Peter R. Marsh, Montclair,

N. J., assignor to Driver- Harris Company, Harrison, N. .L, acorporation of New Jersey No Drawing. Application February 10, 1956Serial No. 564,614

4 Claims. (Cl. 75-171) This invention relates to alloys, and moreparticularly to an alloy of nickel and chromium, containing appreciableamounts of aluminum and appreciable amounts of silicon, and a method ofmaking the alloy.

One object of the invention is to produce an alloy which has a higherspecific resistance than the standard nickel'chromium andnickel-chromium-iron alloys, and a higher resistance than the standardlow temperature coefiicient of resistance alloys for use in lowtemperature resistors. Another object is to produce a heating elementalloy having an improved life at standard testing temperatures and alife above these standard temperatures equivalent to the 80 nickel-20chromium alloy at the standard temperatures.

A number of alloy systems, while having the property of potentially highresistivities, are found to involve extreme difliculties in theirworking and processing due to low ductility and non-malleabilitycharacteristics, and also are known to have the undesirable property ofconsiderable growth when used at high temperature.

J. M. Lohr, in his U. S. Patent No. 2,533,735, has shown that when threepercent of aluminum was added at the expense of an equivalent amount ofnickel in the ductile and non-grown 80-nickel-20 chromium alloy, theresulting specific resistance, after a rapid 1700 to 2000 F. anneal andair quench, is increased from 640 ohms/c. m. f. for the basic 80nickel-20 chromium alloy to 740 ohms/c. m. f. for the 77 nickel-20chromium-3 aluminum alloy. By following the heat treatment as disclosedby Lohr, but in a temperature range of 900 to 1200 F. for 1 to 20 hoursthe latter resistance was increased to approximately 800 ohms/c. m. f.Although such an alloy is applicable for low temperature resistors withthe property of a negligible temperature coeflicient of resistance, thealloy is not suitable as a high temperature heating element. In order togo to higher resistivities and to maintain heating element qualities,higher aluminum and silicon additions were tried to accomplish theobjective of this invention. Although the melting point is lowered bysuch additions, the alloy can be operated nearer to its melting pointwithout early failure due to the formation of selective but complexoxides of aluminum, silicon, and chromium forming an adherent andprotective oxide film for enhancing the life at temperature. However,when attempts are made to produce such an alloy, the same undesirableproperties common to the other high resistivity alloy systems, namely,relative non-forgeability and non-malleability, were encountered.

Melts containing a minimum of 4% aluminum and 1.3% silicon produced wirehaving a resistivity of about 840 ohms/c. m. f. which, upon response toheat treatment as disclosed by Lohr, rose to around 950 ohms/c. m. f.Attention then was focused on additions of 5% aluminum and /z% silicon,as it was determined that further additions of these elements results inan alloy having at least 1000 ohms/c. m. f. These experiences, however,made it obvious that there is a critical limit of total aluminumandsilicon additions after which nonmalleability occurs and which actsas a barrier to obtain desirable yields of t the higher resistivitymaterial, unless "satisfactory mel-ting and forging processes were foundto produce sound alloy ingots and bars.

' In order to eliminate non-rnalleability and increase the yield, themelt was fully deoxidized and degassified before the additionof-aluminum and silicon so as to minimize the formation of alumina andsilica. In this manner :sound ingots were produced which, on forging,have a workingrange of 2000 to 2300 F. The only defects found were, insome cases, the formation of edge checks which were subsequentlyeliminated by the addition of small amounts of rare earth metals to theladle, the addition being approximately 0.1%.

The details for the successful melting and forging of alloys consistingof 4 to 6% aluminum, 1 to 2% silicon, 20% chromium, 0.03 to 0.25%carbon, 1% maximum of manganese, residual amounts of magnesium and rareearths, and balance essentially nickel, are as follows:

All of the nickel and chromium are melted down under a ground glass limeslag. When all of the metal is melted, a dry dross forms which isskimmed oif with the slag. Carbon is then added in small amounts at atime until the bath is quiet and a porosity test shows no signs of gas.The temperature of the bath is then adjusted to allow for the exothermicreaction caused by the next addition so that the bath does not overheat.At this point the mag nesium is added, which effectively deoxidizes thebath. Immediately prior to tapping the furnace the aluminum and siliconare added and just before the melt is poured from the ladle the rareearth addition is made. The forging is carried out starting at 2300" F.and continuing to 2000 F., light blows being employed. at all times. Therolling to rod or strip is accomplished in the conventional manner.

Examples of life tests and analyses of the alloy as compared to theconventional nickel-20 chromium alloy type are as follows:

An alloy containing 0.04% carbon, 0.30% manganese, 1.08% silicon, 19.52%chromium, 4.92% aluminum, balance essentially nickel, had a useful lifeof 404 hours when tested at 2150 F., the temperature employed in testingthe 80-20 nickel-chromium alloy.

Alloys containing 1.30% silicon, 4.27% and 5.2% aluminum and the otherelements substantially in the same percentages as stated above showed auseful life of 325 hours and 315 hours, respectively. As compared tothese results the useful life of an 80-20 alloy, when tested at 2150 F.is of the order of 200 hours.

By means of the above disclosed procedure, a workable electrical heatingand resistance alloy is produced, having a higher specific resistance,higher life at standard heating temperatures, and higher operatingtemperature than can be accomplished by using the conventional 80nickel-2O chromium alloy and without encountering undesirable growthcharacteristics.

While I have described the alloy as consisting essentially of theenumerated elements with the balance essentially nickel, the presence ofsmall amounts of other elements, originally added to the melt asdeoxidizers or present as impurities in one of the elements of thealloy, in amounts which have no appreciable effect on the properties andcharacteristics of the resulting alloy is to be considered to be withinthe scope of the invention.

1 claim 1. A ductile, low-growth, high electrical resistance andnegligible temperature coefficient of resistance alloy consistingessentially of 15% to 25% chromium, 4% to 6% aluminum, 1% to 2% silicon,0.03% to 0.25%

nickel.

2. A ductile, low-growth, high electrical resistance and negligibletemperature coeflicient of resistance alloy consisting essentially ofsubstantially 20% chromium, 4% to 6% aluminum, 1% to 2% silicon, 0.03%to 0.25% maximum carbon, 1% maximum manganese, balance essentiallynickel.

3. A ductile, low-growth, high electrical resistance and negligibletemperature coefficient of resistance element consisting essentially of15% to 25% chromium, 4% to 6% aluminum, 1% to 2% silicon, 0.03% to 0.25%carbon, 1% maximum manganese, balance essentially nickel.

4. A ductile, low-growth, high electrical resistance and negligibletemperature coefficient of resistance element 15 H consistingessentially of substantially 20% chromium, 4% to 6% aluminum, 1% to 2%silicon, 0.03% to 0.25 maximum carbon, 1% maximum manganese, bal' anceessentially nickel.

References Cited in'the file of this patent UNITED STATES PATENTS2,174,919 Kay Oct. 3, 1939 10 2,304,353 Grifiiths Dec. 8, 1942 2,460,590Lohr Feb. 1,1949

2,533,735 Lohr Dec. 12, 1950 FOREIGN PATENTS Australia Nov. 13, 1941

1. A DUCTILE, LOW-GROWTH, HIGH ELECTRICAL RESISTANCE AND NEGLIGIBLETEMPERATURE COEFFICIENT OF RESISTANCE ALLOY CONSISTING ESSENTIALLY OF15% TO 25% CHROMIUM, 4% TO 6% ALUMINUM, 1% TO 2% SILICON, 0.03% TO 0.25%CARBON, 1% MAXIMUM MANGANESE, BALANCE ESSENTIALLY NICKEL.